Exercises¶

• “What is the length of all streets in New York, as measured in UTM 18?”

  SELECT Sum(ST_Length(geom))
    FROM nyc_streets;
  

  10418904.7172
  

• “What is the WKT definition of SRID 2831?”

  SELECT srtext FROM spatial_ref_sys
  WHERE SRID = 2831;
  

  Or, via prj2epsg

  PROJCS["NAD83(HARN) / New York Long Island",
    GEOGCS["NAD83(HARN)",
      DATUM["NAD83 (High Accuracy Regional Network)",
        SPHEROID["GRS 1980", 6378137.0, 298.257222101,
          AUTHORITY["EPSG","7019"]],
        TOWGS84[-0.991, 1.9072, 0.5129, 0.0257899075194932, -0.009650098960270402, -0.011659943232342112, 0.0],
        AUTHORITY["EPSG","6152"]],
      PRIMEM["Greenwich", 0.0,
        AUTHORITY["EPSG","8901"]],
      UNIT["degree", 0.017453292519943295],
      AXIS["Geodetic longitude", EAST],
      AXIS["Geodetic latitude", NORTH],
      AUTHORITY["EPSG","4152"]],
    PROJECTION["Lambert Conic Conformal (2SP)",
      AUTHORITY["EPSG","9802"]],
    PARAMETER["central_meridian", -74.0],
    PARAMETER["latitude_of_origin", 40.166666666666664],
    PARAMETER["standard_parallel_1", 41.03333333333333],
    PARAMETER["false_easting", 300000.0],
    PARAMETER["false_northing", 0.0],
    PARAMETER["scale_factor", 1.0],
    PARAMETER["standard_parallel_2", 40.666666666666664],
    UNIT["m", 1.0],
    AXIS["Easting", EAST],
    AXIS["Northing", NORTH],
    AUTHORITY["EPSG","2831"]]
  

• “What is the length of all streets in New York, as measured in SRID 2831?”

  SELECT Sum(ST_Length(ST_Transform(geom,2831)))
    FROM nyc_streets;
  

  10421993.706374
  

  Note

  The difference between the UTM 18 and the Stateplane Long Island measurements is (10421993 - 10418904)/10418904, or 0.02%. Calculated on the spheroid using Section 17: Geography the total street length is 10421999, which is closer to the Stateplane value. This is not surprising, since the Stateplane Long Island projection is precisely calibrated for a very small area (New York City) while UTM 18 has to provide reasonable results for a large regional area.

• “What is the KML representation of the point at ‘Broad St’ subway station?”

  SELECT ST_AsKML(geom)
  FROM nyc_subway_stations
  WHERE name = 'Broad St';
  

  <Point>
    <coordinates>
      -74.010671468873468,40.707104815584088
    </coordinates>
  </Point>

  Hey! The coordinates are in geographics even though we didn’t call ST_Transform, why? Because the KML standard dictates that all coordinates must be in geographics (ESPG:4326, in fact) so the ST_AsKML function does the transformation automatically.